Two beeps is generally delayed charging. See 217-219 in the (2018) Volt Owner's Manual. Also See Programming Charging from Page 123.Lately, most of the time when I plug in my car, either to my 120v cable or my L2 charger, the car will beep twice and the charging light will stay on steady and the car won’t charge.
But then I unplug the car and immediately plug it back in and it charges normally. Any ideas what this is?
The charging light will "stay on steady", what do you mean? If delayed charging it should be a "long flashing green" for the main charging light on the top of the dash. Don't know how the charging port light would be, if that's what you mean.
When you have a delay, a quick unplug-plug (under 5 seconds) tells the car you really want to charge and it charges. See Page 215 in the 2018 Owners Manual.
I have a 2013 and it's basically the same. I have time-of-use charging at home so use delayed charging, but just occasionally I'm at a friend's house on a weekday evening and have to do the unplug-plug to get it to charge.